Soil and groundwater environments are an important part of the ecosystem. In recent years, with the global industrialization process, China's soil contamination has become increasingly serious, wherein organic contamination has become one of the important types of contamination, which not only destroys the ecological environment itself, but directly or indirectly endangers the environment and human health and safety. More seriously, volatile and semi-volatile organic contaminants migrate to deep saturated regions (aquifers) in unsaturated regions (aeration zones), which especially threatens the groundwater resources with serious consequences.
At present, the commonly used soil and groundwater remediation engineering techniques are divided into two methods: in-situ remediation and ex-situ remediation. Ex-situ remediation is easy to cause leakage and spread of contaminants due to its correlation to excavation, transportation, remediation of different places for contaminated soil and pump & treat of groundwater. The important reason for the in-situ remediation technology being favored is that it can solve the problem of deep soil and groundwater contamination, avoiding problems such as deep foundation pit excavation, precipitation and secondary pollution, and greatly improving safety and economy.
Since the mid-1980s, the United States has invested heavily in soil and groundwater remediation, and some new in-situ remediation technologies have emerged. In-Situ Chemical Oxidation (ISCO) technology adds chemical oxidants to soil and groundwater to oxidize contaminants in groundwater to relatively less toxic products (such as carbon dioxide, water, or chloride ions) for the purpose of remediation. This type of technology can process multiple contaminants at the same time, and the treatment efficiency is high, and chemical oxidation is generally not limited by the concentration of contaminants. Commonly used chemical oxidizing agents include Fenton's reagent, potassium permanganate, ozone, activated persulfate, and the like, which can remedy organic pollutants such as benzene, nitrobenzene and petroleum hydrocarbons in soil and groundwater. Factors affecting in-situ chemical oxidation techniques include: the diffusion of oxidants in groundwater, the type of oxidant, the amount of oxidant added, the type and concentration of contaminants, and so on.
In-situ injection requires the design of two important parameters: a) injection points (drilling holes) distribution manner and parameters (mechanical cost); b) design of agent dosing ratio (agent cost). The design of the agent dosing ratio is one of the most critical design parameters for in-situ chemical oxidation remediation.
Due to the peculiarities of the physicochemical properties of volatile/semi-volatile (VOCs/SVOCs) organics, they are often present in the soil and groundwater in the states of adsorption, dissolved state, gas phase (relative proportion of saturated soil layer is small), and free phase, and there is a certain degree of migration and transformation therein. Therefore, the distribution of these organic contaminants in different formation is poor in uniformity. Considering that most organic contaminants have certain solubility, they have a certain distribution in groundwater. The migration law of contaminants in soil and groundwater is related to soil organic matter content, soil permeability, soil structure and properties (such as soil density, compactness, porosity), characteristics of soil layer stratification (such as partial inclusion of thin interlayer or lens body), characteristics of contaminants (solubility, vapor pressure, Henry's constant, soil-water distribution coefficient, etc.).
According to the Technical Guidelines for Site Environmental Investigation (HJ25.1-2014), the most basic requirements for the original site survey sampling unit are designed to be no more than 1600 m2 (40 m×40 m grid). The specification of the unit is far from enough for the design of the post-remediation project, plus the factors such as insufficient attention and cost constraints in the previous site investigation, the input work amount in the investigation stage is small, and the large-scale soil and groundwater remediation project is lacking in the contamination data obtained from the original site investigation, so that it is difficult to guide the remediation and refinement construction. This is often insufficient to support the optimization design of the in-situ chemical oxidation engineering process parameters of the remediation project, resulting in pressures and risks in the construction period, cost, etc. In particular, for construction in heavily contaminated regions, the dosing design of agent has great remediation risks and economic risks. Due to the complexity of hydrogeology, and differences in concentration and distribution of organic contaminants, it is urgent to identify the concentration partition spatially for key plots/regions of large contaminated sites through density-increased supplementary investigation thereby guiding the optimization of engineering remediation design parameters.
There are two main ways of in situ dosing for in-situ chemical oxidant remediation agents: agitation and infusion/injection, wherein in-situ infusion/injection is divided into: Geoprobe drill bit high pressure injection, well-building injection (PVC injection well), Chemgrout grouting technology, In-situ deep mixing grouting technology and other geotechnical grouting techniques.
U.S. Pat. No. 5,639,182, “Soil In-situ Remediation Method” relates to an in-situ soil mixing method, wherein a mobile crawler rig equipped with a vertically driven impeller bit that allows for in-situ mixing of contaminated soil. The mixing drill bit employed is an integrated impeller bit, so the mixing depth is limited and the resistance is large. The application has certain limitations and is only suitable for the treatment of looser formations. It is difficult to achieve adjustment of the agent dosing parameters of vertical depth by the mixing technique.
In-situ drill bit direct pressure injection is to inject the prepared agent into the contaminated soil layer through the injection drill bit under a certain pressure and continuously mix uniformly the contaminated soil with the oxidizing agent to remedy the contaminated soil and groundwater. Two chemically oxidized in-situ drill injection remediation systems are disclosed in U.S. Patent No. US 2002/0143226 A1 and U.S. Pat. No. 6,457,905. Both remediation systems inject chemical oxidants into contaminated soil through augers or injection drill bits.
Well-building injection technique: in Chinese invention patents such as Application No. 201410387735.4 “An organic contaminated soil and groundwater in situ remediation device and remediation method”, Application No. 201410615166.4 “An in-situ chemical oxidation injection device for the remediation of contaminated soil and groundwater”, a well-building injection in-situ chemical oxidation remediation technique is mentioned, and a remediation method for the injection well in the remediation area is required before the remediation. These two inventions use the intermittent injection method, which can not control and realize the optimal design and implementation of the dosing ratio of the remediation agent in the vertical depth direction of the soil layer.
A chemical oxidation injection well injection system is disclosed in U.S. Patent No. US 2003/0069142. The device is provided with a baffle in front of the screen tube containing the solid powder agent, and after the agent in the screen tube dissolves, remediation for the contaminated soil and groundwater is achieved as the groundwater flows to the target area through the baffle. A chemical oxidation in-situ injection well remediation technique is disclosed in U.S. Patent No. US 2008/0174571 A1. The remediation system injects hydrogen peroxide, ozone and compressed air into the ground through injection wells to remedy contaminated soil and groundwater. Both of these inventions make it difficult to control the amount of agent dosing amount in the vertical depth direction.
In the Chinese invention patent of “a chromium-contaminated in-situ remediation method” of application number 201510159423.2, the agent dosing parameters are obtained according to parameters such as the overlap ratio (the ratio of the area overlapped between the adjacent injection points and the coverage area S of the injection point) calculated by the coverage area of the injection dilling hole, and the number of hole distribution in the injection point. The disadvantage of this calculation method is that the overlap ratio parameter cannot be measured in the engineering implementation, and the agent dosing parameter is not considered to be optimized in the vertical depth direction.
In the Chinese invention patent of “Method for removing chlorine olefins in water by in-situ chemical oxidation”, application number 201410148583.2, the use of potassium permanganate to remove organic compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE) in water with the aid of surfactants is mentioned. Although the removal rate is high, the engineering operability is poor and it is not suitable for the combined contamination of soil and groundwater.
The application process of the above remediation technology requires a series of in-situ remediation systems and processes to optimize the dosage of the remediation agent.